This invention relates in general to holders or cassettes for film. More specifically it relates to a cassette that holds large format film units of the self-developing type for exposure to x-rays.
In large format radiography (e.g. 8.times.10 or 10.times.12) it is standard practice to load the film into a holder or cassette. The cassette, which is typically light tight when loaded with film, facilitates storage, handling and processing of the film.
Cassettes have proven to be particularly useful in radiography using self-developing film. In medical applications, for example, self-developing radiographs are useful in delicate operations where the progress of a catheter through blood vessels must be repeatedly monitored. Daylight pre-loading of the cassette with the film, handling convenience, and relatively short time periods to prepare the cassette for exposure and processing are all highly desirable qualities. For x-ray applications, the cassette preferably includes a rigid screen or other surface that has a layer of phosphors that emit visible light when exposed to x-rays. The photosensitive layer of the film is held in close and continuous contact with the phosphor layer so that the x-ray induced light of the phosphors intensifies the image to be recorded on the film.
Cassettes heretofore known in the art, for example, U.S. Pat. Nos. 2,709,223 to Bachelder et al and 3,103,864 to the present applicant, are characterized by housings that are hinged to open in the manner of a book. The film is loaded in the opened cassette and an intensifying screen is placed opposite the photosensitive layer of the film, preferably by pivoting the screen about one edge. The cassette is then closed and latched to render it light-tight.
Both of these cassettes also utilize a spring-biased, plate-like portion of the housing to grip the film unit. This gripping action secures the film in a predetermined position and places its photosensitive layer in a flat configuration coincident with a film plane. In radiography, this action also places the photosensitive layer in continuous, intimate contact with the phosphor layer. These cassettes also have manually operated cam levers that draw the movable housing plate away from the film against the spring force. The cam action allows the sliding removal of an opaque envelope that shields the photosensitive layer from ambient light before and during the loading operation. After exposure, the cam action allows the film unit to be withdrawn from the cassettes through an end opening for processing. It should be noted that these cassettes are designed for use with a film unit that includes both a "negative" photosensitive layer and a "positive" image receiving layer, together with suitable support layers and a rupturable pod of processing fluid as described in the aforementioned patents.
While these cassettes function adequately, they suffer from various disadvantages. First, they are relatively inconvenient to use. Film loading requires opening the cassette and manually positioning the film in the cassette with an interposed intensifying screen. With the Erikson '864 cassette, for example, one housing member must be pivoted twice through 360.degree. and latched each time. Second, because the cassettes are fully opened to load the film, the intensifying screen and other surfaces are subject to contamination by fingerprints or dust. Also, the negative is more susceptible to fogging by exposure to the afterglow of certain phosphors. Third, the relatively heavy springs utilized in these cassettes can cause a highly undesirable outward bowing of the housing. This situation is particularly characteristic of the earlier Bachelder cassette where the central bowing of the housing plate under the springs can be as much as one-eighth inch. Another problem of the Bachelder cassette is that it is comparatively heavy.
A more recent cassette manufactured by the Polaroid Corporation is similar to the Erikson '864 x-ray cassette, but with modifications for conventional photography using a photosensitive or "negative" film unit sold by Polaroid Corporation under the trade designation "Type 808, 8.times.10 Land Film." This cassette also opens to load the film unit. A plastic tab engages a flap on the film unit to register the film. The cassette is closed, and removal of an opaque envelope and a dark slide mounted in the cassette present the film for exposure. After exposure, the dark slide is reinserted and a positive, image receiving film unit is inserted into the cassette in superposed relationship with the negative. These mated film elements are then pulled together through a pair of processing rolls.
While the cassette is suitable for use with modern self-developing film where the negative and positive elements are not physically linked together, it suffers from disadvantages of the earlier cassettes in that it must be opened for loading. In addition, since there is no system for relieving the pressure on the various sliding surfaces, the friction accompanying the removal and insertion of the dark slide can move the camera or shift the film within the cassette. Another disadvantage is that the high friction associated with the sliding insertion of the positive film unit introduces a significant time delay between exposure and processing.
U.S. Pat. No. 3,330,953, also to the present applicant, describes another x-ray cassette adapted for analysis of crystals by back reflection laue exposure. In this cassette the film is inserted through an end opening of the cassette so that the intensifying screen is not exposed to contaminants and light. Also, the cassette has a slide member with cam surfaces that control the grip and release of the film unit. This cassette, however, differs markedly from the aforementioned construction since it is adapted for use with film of much smaller format, typically 4.times.5. In particular, the pressure applying arrangement to place the crystal, film and screen in intimate contact employs a thin, flexible light barrier that extends across a large exposure window formed in the housing.
It is therefore a principal object of this invention to provide a cassette for large format, self-developing film units that is highly convenient to use and does not expose internal components to contaminants during loading.
Another object of the invention is to provide a cassette that allows speedy, uncomplicated daylight loading with a minimum likelihood of fogging.
Yet another object of this invention is to provide a cassette with the foregoing advantages that provides automatic and optional manual release of a pressure applying member that holds the film unit in a precisely predetermined flat configuration against an intensifying screen suitable for radiography.
Still another object is to provide a cassette that is lightweight, accommodates modern self-developing film and yields good quality prints.